Help for an Ailing Univox U1040

The U1040’s back panel includes jacks for
both reverb and tremolo footswitches.

Hi Jeff,
It’s probably a lingering artifact from
some emotional turbulence years ago,
but I’m fond of the goofy, early-’70s,
two-tone blue Univox amps with the
orange logos. I bought a weary, nonworking
U1040 project off Craigslist
and have been trying to nurse it back
to health. A local shop got it to pass
signal and technically function, but
with four power tubes, some big-bruiser
transformers, and a rating of 105
watts, I was expecting more. The amp’s
volume seems weak and muted, its
tone is nothing special, and the whole
thing just seems sonically constipated. I’ve been told this amp has an
unusual design. What remedy might
the amp doctor prescribe for this lackluster
combo?Best regards,
Tom Shaw

Hi Tom,
Although I’ve never owned one, I absolutely
remember these cool and unusual-looking
amps. According to the Univox site, your
1040 Quad Reverb was one of a large contingent
of models designed and marketed
in 1971. Although Univox was originally
a Japanese company, around the time your
1040 was produced, these amps would have
probably been assembled in Westbury, New
York, using Japanese-built chassis. This model
was also available with a 4x10 speaker configuration
(U1044). These amps sold for $480
at the time, which was a pretty decent chunk
of change considering that a Volkswagen
Beetle was selling for under $2,000.

Although this 1040 model exists, apparently
its schematic does not—at least online.
So as a reference, I chose a schematic for
another model with the same basic specs, the
2-channel 1010. It has 10 tubes, a solid-state
power supply, reverb, tremolo, presence, and
a 105-watt RMS output. My guess is this
will be almost identical to your model.

When troubleshooting older amps with
no known history, it makes sense to start
with the tubes. Used or abused output
tubes can certainly leave an amp with
lackluster performance. Install another set
of 6L6s and see if this makes a substantial
difference. If so, have the new tubes biased
properly for optimal performance. I’d also substitute each preamp tube and listen for
an improvement. This may be a case where
each tube replacement brings the amp
another step towards proper performance.

While there are a few atypical design elements,
there’s really nothing too “unusual”
about it. The most noticeable difference is
that the unit uses a 6AN8 triode/pentode
tube in the reverb circuit. The pentode half
of this tube feeds the triode half, which is
basically a 12AU7, and the triode side drives
the reverb tank. A little out of the ordinary,
but certainly not an area that should be causing
amp anemia. The following design differences,
however, could be possible suspects.

The power supply differs from what’s
normally found in the majority of guitar
amps. In most guitar amplifiers, the power-supply
voltages that are fed to different
stages of the amplifier are all derived, in
one way or another, from the main supply
voltage, traditionally called B+. But in this
design, the power supply is actually split
into two discrete sections. The full power
supply (650V DC) is feeding the plates
of the output tubes, while the rest of the
amplifier is fed with exactly one half of that
voltage (325V DC), which is sourced at the
center of the power-supply filter stack.

Most Music Man amps used a very
similar design, but since the preamp stages
of the Music Man were all solid-state, the
half power-supply voltage was only used
to supply the screen grid voltage to the
output tubes, and here is where a potential
problem could exist in your amp. The half
voltage is derived at the center point of
two series capacitors. If these capacitors are
worn, dried out, and out of balance, the
half voltage could be substantially low and
cause weak output from the output tubes,
as well as lower gain in the preamp stages.

Check the half voltage. If it’s substantially
low, replacing these two 100 μF 450V
capacitors could bring the amp back to life.
And while you’re at it, I’d replace all of the
filter caps, as their performance could be
questionable as well.

Next, I’d look at the output stage. In
most designs, the plates of each pair of
output tubes on either side of the output
transformer are connected together. In
this design however, the plates are separated
by a 100 Ω resistor. If those resistors
have failed, you may only be getting
output from two of the four output tubes.
Replacing those resistors should enable all
four tubes to operate again.

If none of the above potential causes are
the source of the lackluster performance, I’d
finally suggest looking at the speakers. Over
the years I’ve seen many instances where
the original speakers in vintage amps can
become weak and worn out, and the amp
sounds and feels completely underwhelming.
Disconnect the internal speakers and play
the amp through a good extension cabinet.
New speakers can make all the difference in
the world.

Warning: All tube amplifiers contain lethal
voltages. The most dangerous voltages are
stored in electrolytic capacitors, even after the
amp has been unplugged from the wall. Before
you touch anything inside the amp chassis, it’s
imperative that these capacitors are discharged.
If you are unsure of this procedure, consult
your local amp tech.

Jeff Bober is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at
pgampman@gmail.com.

Jeff Bober is one of the godfathers of the low-wattage amp revolution. He co-founded and was originally the principal designer for Budda Amplification, though he launched EAST Amplification (eastamplification.com) in 2010. You can catch his podcasts at ampsandaxescast.com or email him at pgampman@gmail.com.

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